The present invention is in the field of plasma torches, which are generally used for cutting, welding and spray bonding, and is specifically directed to a height acquisition and maintenance control system for such a torch.
Plasma torches, otherwise known as electric arc torches, are known in the art for performing operations, such as cutting, welding, etc., on workpieces, and operate by directing a plasma consisting of ionized gas particles towards a workpiece. An example of the conventional single gas plasma torch is illustrated in Hatch, U.S. Pat. No. 3,813,510, assigned to the assignee herein. As the latter patent illustrates, a gas to be ionized, such as nitrogen, is fed through channels in the torch mechanism in such a manner as to swirl in front of the end of a negatively-charged electrode. The welding tip which is adjacent the end of the electrode has a sufficiently high voltage applied thereto to cause a spark gap to jump between the electrode and the welding tip, thereby heating the gas and causing it to ionize. A pilot DC voltage between the electrode and the welding tip maintains an arc known as the pilot or non-transferred arc. The ionized gas in the gap appears as a flame and extends externally off the tip where it can be seen by the operator. The extension of the pilot arc and the flame, which for practical purposes may be considered as being co-extensive, depends upon the power in the gap, i.e., the arc current, as well as the pressure of the gas forced into the gap and out of the torch. The non-transferred arc provides a source of light which enables the operator to see the proper position for the torch before starting the welding or cutting operation. In actual practice, when the non-transferred arc is on, a loop-shaped arc extending out of the torch can be seen. As the torch head is brought down towards the workpiece, the arc jumps from the electrode to the workpiece due to the fact that the impedance of the workpiece current path is lower than the impedance of the welding tip current path.
The quality of the cut in the workpiece depends upon several variables, e.g., arc current, type of metal, thickness of metal, and torch height above the metal. For a given workpiece, the current and height are set to determine the quality of the cut. In the past, it has been conventional to set a plasma torch at the desired height, typically about 1/4" above the workpiece, by manually pulling down the torch assembly and physically measuring the distance. Thereafter, the torch was held at this height during the entire cutting operation.
With the advent of numerical machine control of torch cutting patterns, it is not uncommon to have assemblies which are so large that it is impractical for the operator to initially manually adjust the torch height. The operator has to climb over the work table to make the adjustment.
Furthermore, there are several reasons why it is undesirable to fix the position of the torch and leave it at that position during the entire cutting sequence. For plasma torches, a nominal height above the workpiece, i.e., standoff, is 1/4" with a permissible variation of 1/8". The work tables are not perfectly flat, and for very long workpieces, a small divergence from perfect flatness can create a significant difference in the standoff heights at the opposite ends of the workpiece. A warped workpiece compounds the problem. Also, there are cases where it is necessary to cut a corrugated workpiece. Traditionally, the height and arc current are set to cut through the valleys. However, this causes the cut in the peaks to be too wide.
In view of the above disadvantages of maintaining the torch at a fixed height position, there have been several attempts in the prior art to provide an apparatus for automatically controlling the torch height during the cutting operation to maintain the standoff height at a constant value. There have also been attempts to position the torch at a desired standoff height prior to cutting, welding or spray bonding, other than by the manual method mentioned above. However, these techniques and apparatus have not completely solved the above problems or have introduced problems of their own.
Both mechanical and electrical sensing devices have been used to maintain a fixed height above the workpiece once the torch is initially set. The mechanical sensing device uses one or more rollers attached to and extending downwardly and outward from the torch. The roller rides on the workpiece, and variations in height relative to the torch are sensed and fed to a torch height control motor. Problems with this technique are that the sensors are a finite distance away from the torch and, therefore, are not adequate for a workpiece such as corrugated metal, the sensors are bulky, and they become dirty from the cutting debris and can fail to operate. Also, if only a single sensor is used and placed in the direction of movement, it will give a false signal when the torch approaches the edge.
Capacitive and inductive sensors have also been proposed. These sensors are placed around the torch and sense a capacitive or inductive change as the distance between torch and workpiece changes. The capacitors or inductors are part of an oscillator which changes frequency in response to changes in capacitance or inductance. The frequency changes are detected and used to control the torch height control motor. Such sensors are bulky, and incorrect operation can result when cutting debris splashes onto the sensors. Also, electrical noise caused by the arc can interfere with the accuracy of operation, and heat from the arc can damage the sensors. A water shield, which surrounds the arc in many applications, can interfere with the correct operation of the sensors.
Arc voltage sensing during cutting and welding has also been used to maintain the torch at a fixed height above the workpiece. This technique is possible due to the fact that the arc voltage, which can be measured between the torch and the workpiece, is a function of several variables, including torch height above the workpiece, type and thickness of the metal workpiece, torch parts, horizontal travel speed, gas pressure and arc current. Since plasma torches include current regulators to regulate torch current, for a given workpiece, all variables, except torch height, can be and typically are kept constant. Thus, variations in the height above the workpiece can be measured by variations in the arc voltage, which is then used to control the torch height control motor.
While the latter system is an improvement over electrical and mechanical sensing, it only regulates height once it is properly set. The system cannot be used to regulate height until after the cutting arc has started and stabilized and a cut is made. This is because the arc voltage will have transients until the arc stabilizes and will not reach a final "regulating" value until a cut is made. Also, for any given cut, it is necessary to provide a reference arc voltage against which the actual arc voltage is compared. Thus, it must be determined a priori what arc voltage corresponds to the desired torch height above the work plate.
The set-up procedure for such a system can be based on arc voltage values taken from a manufacturer's table provided for that purpose, or can be based on the arc voltage observed during a test cut. The manual includes appropriate arc voltages for several torch heights, workpiece types, gas flow, etc. The operator would dial in the arc voltage as the reference voltage. A test cut should be used even if the manual is consulted to insure the accuracy of the reference arc voltage.
In either case, the torch height must initially be set using a ruler or a spacer block, or a gas back pressure system. The gas back pressure system initially sets the torch height by measuring the back pressure of the gas in the feed lines. While this method has proved useful, it is limited in its application and does not simplify the setting procedure for subsequent height regulation. It is limited to use with plasma torches that have a large diameter gas feed and do not have a restrictive orifice at the end of the torch. Thus, a substantial number of plasma torches cannot use this method.
The gas back pressure measuring system also cannot be used when either the non-transferred arc or transferred arc is on. This is because the arc varies the gas pressure and the cut would change the back pressure dramatically.